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RESPIRATORY SYSTEM

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Originally appearing in Volume V23, Page 188 of the 1911 Encyclopedia Britannica.
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RESPIRATORY SYSTEM. (I). ANATOMY—The respiratory tract consists of the nasal cavities, the pharynx, the larynx, the trachea, the bronchi and the lungs, but of these the two first parts have been treated in separate articles (see OLFACTORY SYSTEM and PHARYNX). The larynx is the upper part of the air tube which is specially modified for' the production of notes of varying pitch, though it is not responsible for the whole of the voice. Its frame-work is made up of several cartilages which are moved on one another by muscles, and it is lined internally by mucous membrane which is continuous above with that of the pharynx and below with that of the trachea or windpipe. The larynx is situated in the front of the neck and corresponds to the fourth, fifth and sixth cervical vertebrae. For its superficial anatomy see ANATOMY, Superficial and Artistic. The thyroid cartilage (see fig. I) is the largest, and consists of two plates or alae which are joined in the mid-ventral line. At the upper part of their junction is the thyroid notch and just below that is a forward projection, the pomum Adami, best marked in adult males. From the upper part of the posterior border of each aia the superior cornu rises up to be joined to the tip of the great cornu of the hyoid bone by the lateral thyro-hyoid ligament, while from the lower part of the same border the inferior cornu passes down to be fastened to the cricoid cartilage by the crico-thyroid capsule. From the upperborder of each ala the thyro-hyoid membrane runs up to the hyoid bone, while near the back of the outer surface of each the oblique line of the thyroid cartilage runs downward and forward. The cricoid cartilage (see figs. 1 and 2) is something like a signet ring with the seal behind; its lower border, how-ever, is horizontal. To the mid-ventral superior part of its upper tubercle on the border is attached car t ghyroid cartilage the mesial part of Oblique line the crico - thyroid membrane, which attaches it to the lower border of the thyroid cartilage though the lateral parts of this membrane pass up in- ternally to the After D. J. Cunningham, from Cunningham's Text-book of Anatomy. forms a pyramid with its apex upward and with an anterior posterior and internal or mesial surface. The base articulates with the cricoid by a concave facet, surrounded by the crico-arytenoid capsule, and the two arytenoids are able to glide toward or away from one another, in addition to which each can rotate round a vertical axis. From the front of the base a delicate process projects which, as it is attached to the true vocal cord, is called the vocal process, while from the outer part of the base another stouter process Cartilago triticea Thyro-epiglottidean ligament Superior cornu of thyroid cartilage Cartilage of Santorini Arytenoid cartilage Muscular process of arytenoid cartilage TNT; "' ic^t ~ thyroid cartilage After D. J. Cunningham, from Cunningham's Text-Book and their upper of Anatomy. free edges form the FIG. 1.-Profile View of the Cartilages and true vocal Ligaments of the Larynx. cords. On the summit of the signet part of the cricoid are placed the two arytenoid cartilages (see fig. 2), each of which Inferior tubercle Inferior cornu of thyroid cartilage Crico-thyroid membrane Cricoid cartilage attaches the two,,srico-arytenoid muscles and so is known as the muscular process. Above each arytenoid are two smaller - cartilages known as the cornicula laryngis or cartilages of Santorini and the cuneiform cartilages, but they are not of any practical importance. The epiglottis (see fig. 3), on the other hand, is a very important structure, since it forms a lid to the larynx in swallowing: only the box moves up to the lid instead of the lid moving down to the box. It is leaf-shaped, the stalk (thyro-epiglottid e a n ligament) being attached to the junction of the thyroid cartilages inside the larynx, while the anterior surface of the leaf is closely attached to the root of the tongue and body of the hyoid bone. The posterior Processus vocalis or laryngeal surface is pitted for glands, and near the point where the stalk joins the leaf is a convexity which is known as the cushion of the epiglottis. All the cartilages of the larynx are of the hyaline variety except the epi- glottis, the cornicula laryngis and the cuneiform cartilages, which are yellow elastic. The result is that all except these three tend to ossify as middle age is approached. The muscles of the larynx are: (r) the crico-thyroids, which are attached to the lower border of the thyroid and the ,anterior part of the cricoid, by pulling up which they make the upper part of the signet, with the arytenoids attached to it, move back and so tighten the vocal cords. (2) The thyro-arytenoids (see fig. 4), which run back from the junction of the thyroid alae to the front of the arytenoids and side of the epiglottis; they pull the arytenoids toward the thyroid and so relax the cords. (3) The single arytenoideus muscle, which runs from the back of one arytenoid to the other and approximates these cartilages. (4) The lateral crico-arytenoids (see fig. 4) which draw the muscular processes of the arytenoids forward toward the ring of the cricoid and, by so doing, twist the vocal processes, with the cords attached, inward toward one another; and (5) the posterior crico-arytenoids (see fig. 4) which run from the back of the signet part of the cricoid to the back of the muscular processes of the arytenoid and, by pulling these backward, twist the vocal processes outward and so separate the vocal cords. All these muscles are supplied by the recurrent laryngeal nerve, except the crico-thyroid which is innervated by the external branch of the superior laryngeal (see NERVES, Cranial). The mucous membrane of the larynx is continuous with that of the pharynx at the aryteno-epiglottidean folds which run from the sides of the epiglottis to the top of the arytenoid cartilages (see (fig. 3). To the outer side of each fold is the sinus pyriformis (see PHARYNX). From the middle of the junction of the alae of the thyroid cartilage to the vocal processes of the arytenoids the mucous membrane is reflected over, and closely bound to, the true vocal cords which contain elastic tissue and, as has After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. *FIG. 4.-Dissection of the Muscles in the Lateral Wall of the Larynx. The right ala of the thyroid cartilage has been removed. true vocal cords is the glottis or rima glottidis. just above the true vocal cords is the opening into a recess on each side which runs upward and backward and is known as the laryngeal saccule; its opening is the laryngeal sinus. The upper lip of this slit-like opening is called the false vocal cord. The mucous membrane is closely bound down to the epiglottis and to the true vocal cords, elsewhere there is plenty of sub-mucous tissue in which the products of inflammation may collect and cause " oedema laryngis," a condition which is mechanically prevented from passing the true vocal cords. In the upper part of the front and sides of the larynx and over the true vocal cords the mucous membrane is lined by squamous epithelium, but elsewhere the epithelium is of the columnar ciliated variety: it is supplied by the superior laryngeal branch of the vagus nerve and above the glottis is peculiarly sensitive. The Trachea or windpipe (see fig. 5) is the tube which carries the air between the larynx and the bronchi; it is from four to four and a half inches long and lies partly in the neck and partly in the thorax: It begins where the larynx ends at the lower border of the sixth cervical, and divides into its two bronchi opposite the fifth thoracic vertebra. The tube is kept always open by rings of cartilage, which, however, are wanting behind, and, as it passes down, it comes to lie farther and farther from the yentral surface of the body, following the concavity of the thoracic region of the spinal column. In the whole of its downward course it has the oesophagus close behind it, while in front are the isthmus of the thyroid, the left innominate vein, the innominate artery and the arch of the aorta. On each side of it and touching it is the vagus nerve. The cervical part of the tube is not much more than an inch in length, but it can be lengthened by throwing back the head. This, of course, is the region in which tracheotomy is performed, and it should be remembered that in children, and sometimes in adults, the great left innominate vein :ies above the level of the top of the sternum. In transverse section the trachea is rather wider from side to side than from before backward. In life the former measurement is said to be about 12.5 mm. and the latter i r mm: It is made up of an external fibro-elastic membrane in which the cartilaginous rings lie, while behind, where these rings are wanting, is a layer of unstriped muscle which, when it contracts, been mentioned, are the upper free edges of the lateral parts of the crico-thyroid membrane. The chink between the two Epiglottis Thyro-hyoid membrane accule of larynx Muscular process of arytenoid cartilage Thyro-arytenoid muscle Thyroid cartilage Crico-arytenoideus lateralis Crico-arytenoideus posticus Crico-thyroid membrane Aryteno-epiglottidean muscle After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. show the outer wall of the right half. Thyroid cartilage Elevation produced by cuneiform cartilage False vocal cord Philtrum ventriculi Elevation produced by arytenoid cartilage Laryngeal sinus True vocal cord Arytenoid muscle Cricoid cartilage Cricoid cartilage draws the hind ends of the rings together and so diminishes the calibre of the tube. Inside these is plentiful submucous tissue Pulmonary artery After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. containing mucous glands and quantities of lymphoid tissue, while the whole is lined internally by columnar ciliated epithelium. The Bronchi (see fig. 5) are the two tubes into which the trachea divides, but, since the branches, which these tubes give off later, are also called bronchi, it may be clearer to speak of primary, secondary and tertiary bronchi. Each primary bronchus runs downward and outward, but the right one is more in a line with the direction of the trachea than the left. The right primary bronchus has also a greater calibre than the left because the right lung is the larger, and for these two reasons when a foreign body enters the trachea it usually enters the right bronchus. The first secondary bronchus comes off about an inch from the bifurcation of the trachea on the right side and, as it lies above the level of the pulmonary artery, it is known as the eparterial bronchus. On the left side the first branch is about two inches from the bifurcation and, like all the remaining secondary bronchi, is hyparterial: the left primary bronchus is therefore twice as long as the right. After the eparterial secondary bronchus is given off the direction of the right primary bronchus is carried on by the hyparterial secondary bronchus,and this, just before reaching the hilum of the lung, divides into upper and lower tertiary bronchi, while the left lower secondary hyparterial bronchus does not divide before reaching the hilum of its lung. Into the hilum or root of the right lung, therefore, three bronchial tubes enter, while on the left side there are only two. The firmly rooted habit of associating the term bronchi with those parts of the main tubes which lie between the bifurcation of the trachea and the point where the first branch comes off makes it very difficult to suggest a nomenclature which calls up any picture of the actual state of things to the mind. Certainly the classification into primary, secondary and tertiary bronchi only goes a very little way toward this, and it should be realized that, call them what we may, there are two long tapering tubes which run from the bifurcation of the trachea to the lower and back part of each lung, and give off a series of large ventral and small dorsal branches. The upper part of each of these long tubes or stem bronchi is outside the lung and in the middle mediastinum of the thorax, the lower part embedded in the substance of the lung. The structure of the bronchi is practically identical with that of the trachea. (See G. S. Huntington's " Eparterial Bronchial System of the Mammalia," Am. Journ. Med. Sci. (Phila. 1898). See also Quain's Anatomy, London, last edition.) The Lungs are two pyramidal, spongy, slate-coloured, very vascular organs in which the blood is oxygenated. Each lies in its own side of the thorax and is surrounded by its own pleural cavity (see COELOM and SEROUS MEMBRANES), and has an apex which projects into the side of the root of the neck, a base which is hollowed for the convexity of the diaphragm, an outer surface which is convex and lies against the ribs, an inner surface concave for the heart, pericardium and great vessels, a sharp anterior border which overlaps the pericardium and a broad, rounded posterior border which lies at the side of the spinal column. Each lung is nearly divided into two by a primary fissure which runs obliquely downward and forward, while the right lung has a secondary fissure which runs horizontally forward from near the middle of the primary fissure. The left lung has therefore an upper and lower or basal lobe, while the right has upper, middle and lower lobes. On the inner surface of each lung is the root or hilum at which alone its vessels, nerves and ducts (bronchi) can enter and leave it. The structures contained in the root of each lung are the branches and tributaries of (1) the pulmonary artery, (a) the pulmonary veins, (3) the bronchi, (4) the bronchial arteries, (5) the bronchial veins, (6) the bronchial lymphatic vessels and glands, (q) the pulmonary plexuses of nerves. Of these the first three are the largest and, in dividing the root from in front, the veins are first cut, then the arteries and last the bronchi. As has been pointed out already, the eparterial bronchus on the right side is above the level of the artery, but all the others (hyparterial) are on a lower level. The bronchial arteries supply the substance of the lung; there are usually two on each side, and they lie behind the bronchi. The blood which they carry is chiefly returned by the pulmonary veins bringing oxidized blood back to the heart, so that here there is a normal and harmless mixture of arterial and venous blood. If there are any bronchial veins (their presence is doubted by some, and the writer has himself carefully but unsuccessfully searched for them several times), they open into the azygos veins of their own side. The bronchial lymphatic vessels lie behind the pulmonary vessels and open into several large glands which are black from straining off the carbon left iii the lungs from the atmosphere. There is an anterior and posterior pulmonary plexus of nerves on each side, the fibres of which are derived from the vagus and the upper thoracic ganglia of the sympathetic. Structure of the Lungs.—As the bronchi become smaller and smaller by repeated division, the cartilage completely surrounds them and tends to form irregular plates instead of rings—they are therefore cylindrical, but when the terminal branches (lobular bronchi) are reached, the cartilage disappears and hemispherical bulgings called alveoli occur (fig. 6 A). At the very end of Part of trachea covered by isthmus of thyroid body Left bronchus Pulmonary artery Ayparterial bronchi each lobular bronchus is an irregular chamber, the atrium (fig. 6 At), and from this a number of thin-walled sacs, about r mm. in diameter, open out. These are called the infundibula (fig. 6 I), and their walls are pouched by hemispherical air-cells or alveoli like those in the lobular bronchi. Each lobular bronchus with its atrium and infundibula forms what is known as a lobule of the lung, and these lobules are separated by connective tissue, and their outlines are evident on the surface of the lung. The muscular tissue, which in the larger tubes was confined to the dorsal part, forms a complete layer in the smaller; but when the lobular bronchi are reached, it stops and the mucous membrane is surrounded by the elastic layer. In the lobular bronchi, too, the lining epithelium gradually changes from the ciliated to the stratified or pavement variety, and this is the only kind which is found in the infundibula and alveoli. Sur-rounding each alveolus is a plexus of capillary vessels so rich that the spaces between the capillaries are no wider than the capillaries themselves, and it is here that the exchange of gases takes place between the air and the blood. Embryology.—The respiratory system is developed from the ventral surface of the foregut as a long gutter-like pouch which reaches from just behind the rudiment of the tongue to the stomach. Limiting the anterior or cephalic end of this is a (1-shaped elevation in the ventral wall of the pharynx which separates the ventral ends of the third and fourth visceral bars and is known as the furcula; it is from this that the epiglottis, aryteno-epiglottidean folds and arytenoid cartilages are developed. Later on the respiratory tube is separated from the digestive by two ridges, one on each side, which, uniting, form a transverse partition. In the region of the furcula, however, the partition stops and here the two tubes communicate. The caudal end of the respiratory tube buds out into the two primary bronchi, and the right one of these, later on, bears three buds, while the left has only two; these are the secondary bronchi, which keep on dividing into two, one branch keeping the line of the parent stem to form the stem bronchus, while the other goes off at an angle. By the repeated divisions of these tubes the complex " bronchial tree " is formed and from the terminal shoots the infundibula bud out. The alveoli only develop in the last three months of foetal life. The thyroid cartilage is probably formed from the fourth and fifth branchial bars, while the cricoid seems to be the enlarged first ring of the trachea. Before birth the lungs are solid and much less vascular than after breathing is established. Their slaty colour is gradually gained from the deposit of carbon from the atmosphere. (For further details see Quain's Anatomy, vol. i., Lond. 1908.) Comparative Anatomy.—It has been shown (see PHARYNX) that in the lower vertebrates respiration is brought about by the blood vessels surrounding the gill clefts. In the higher fishes (Ganoids and Teleosteans) the " swim bladder" appears as a diverticulum from the dorsal wall of the alimentary canal, and its duct (d. pneumaticus) sometimes remains open and at others becomes a solid cord. In the former case it is probable that the blood is to some extent oxidized in the vascular wall of this bladder. In the Dipnoi (mud-fish) the opening of the swim bladder shifts to the ventral side of the pharynx and the bladder walls become sacculated and very vascular, so that, when the rivers are dried up, the fish can breathe altogether by means of it. In the S. American and African species of mud-fish the bladder or lung, as it may now be called, is divided by a longitudinal septum in its posterior (caudal) part into right and left halves. In this sub-class of Dipnoi, therefore, a generalagreement is seen with the embryology or ontogeny of Man's lungs. In the Amphibia the two lungs are quite separate though they are mere sacculated bags without bronchi. A trachea, however, appears in some species (e.g. Siren) and a definite larynx with arytenoid cartilages, vocal cords and complicated muscles is established in the Anura (frogs and toads). In most of the Reptilia the bag-like lungs are elaborated into spongy organs with arborizing bronchi in their interior. From the crocodiles upward a main or stem bronchus passes to the caudal end of the lung, and from this the branches or lateral bronchi come off. The larynx shows little advance on that of the Anura. The respiratory organs of birds are highly specialized. The larynx is rudimentary, and sound is produced by the syrinx, a secondary larynx at the bifurcation of the trachea; this may be tracheal, bronchial or, most often, tracheo-branchial. The lungs are small and closely connected with the ribs, while from them numerous large air sacs extend among the viscera, muscles and into many of the bones, which, by being filled with hot air, help to maintain the high temperature and lessen the specific gravity of the body. This pneumaticity of the bones is to a certain extent reproduced by the air sinuses of the skull in crocodiles and mammals, and it must be pointed out that the amount of air in the bones does not necessarily correspond with the power of flight, for the Ratitae (ostriches and emeus) have very pneumatic bones, while in the sea-gulls they are hardly pneumatic at all. In mammals the thyroid cartilage becomes an important element in the larynx, and in the Echidna the upper and lower parts of it, derived respectively from the fourth and fifth branchial bars, are separate (R. H. Burne, Journ. Anat. and Phys. xxxviii. p. xxvii.). The whole larynx is much nearer the head than in Man, and in young animals the epiglottis is intra-narial, i.e. projects up behind the soft palate. This pre-vents the milk trickling into the larynx during suckling, and is especially well seen in the Marsupials and Cetacea, though evidences of it are present in the human embryo. In the lower mammals an inter-arytenoid cartilage is very frequent (see J. Symington, " The Marsupial Larynx," J. Anat. and Phys. xxxiii. 31, also " The Monotreme Larynx," ib. xxxiv. 90). The lungs show a good deal of variation in their lobulation; among the porcupines as many as forty lobes have been counted in the right lung, while in other mammals no lobulation at all could be made out. The azygous lobe of the right lung is a fairly constant structure and is situated between the post-caval vein and the oesophagus. It is supplied by the terminal branch of the right stem bronchus and, although it is usually absent in Man, the bronchus which should have supplied it is always to be found. (F. G. P.) (2) PHYSIOLOGY So far as is known, the intake of oxygen, either free or combined, and the output of carbon dioxide, are an essential part of the life of all organisms. The two processes are so closely associated with one another that they are always included together under the designation of respiration, which may thus be defined as the physiological process which is concerned in the intake of oxygen and output of carbon dioxide. According to the evidence at present available, it is only within living cells that the respiratory oxygen is consumed and the carbon dioxide formed. The mere conveying of oxygen from the surrounding air or water to these cells, and of carbon dioxide from them to the air or water, is, however, in itself a complex process in the higher animals; and accordingly an account of animal respiration naturally falls into two divisions, the first of which (I.) is concerned with the manner in which oxygen and carbon dioxide are conveyed to and from the living tissues, and the second (II.) with the consumption of oxygen and formation of carbon dioxide by the living tissues themselves. I. In all the more highly organized animals there are special respiratory organs: the lungs in the higher vertebrates; the gills in fishes; the tracheae in insects; and various rudimentary forms of lungs or gills in other higher invertebrates. In the Lob. FIG. 6.-Diagram of Two Lobules of the Lung. B. Bronchus. A. Alveolus. I. Infundibulum. L.E. Lobular bronchus. At. Atrium. Lob. Lobule. present article attention will be specially confined to the case of the higher vertebrates, and in particular to man. Air is brought into the lungs by the movements of breathing (see above, Movements of Respiration). Oxygen from this air passes through the delicate lining membrane of the air-cells of the lungs into the blood, where it enters into loose chemical combination with the haemoglobin of the red corpuscles (see Ba000). In this form it is conveyed onwards to the heart, and thence through the arteries to the capillaries, where it again parts from the haemoglobin, and passes through the capillary walls to the tissues, where it is consumed. Carbon dioxide passes out from the tissues into the blood in a corresponding manner, enters into loose combination as bicarbonate, and possibly in other ways, in the blood, and is conveyed by the veins to the lungs, whence it passes out in the expired air. Pure atmospheric air contains 20.93% of oxygen, '03% of carbon dioxide and 79.04% of nitrogen (with which is mixed about 0.9% of argon). The dried expired air in man contains about 3'5% of carbon dioxide and 17% of oxygen, so that roughly speaking the carbon dioxide is increased by about 3'5% and the oxygen diminished by 4%. Expired air as it leaves the body contains about 6 %, of moisture, compared with usually about 1% in the inspired air. The added moisture and higher temperature of expired air make it decidedly lighter than pure air. Owing to the unpleasant effects often produced in badly ventilated rooms it was for long supposed that some poisonous volatile " organic matter " is also given off in the breath. Careful investigation has shown that this is not the case. The unpleasant effects are partly due to heat and moisture, and partly to odours which are usually not of respiratory origin. The carbon dioxide present in the air of even very badly ventilated rooms is present in far too small proportions to have any sensible effect. The average volume of air inspired per minute by healthy adult men during rest is about 7 litres or •25 cub. ft. In different individuals the frequency of breathing varies considerably—from about 7 to 25 per minute, the depth of each breath varying about inversely as the frequency. During muscular work the volume of air breathed may be six or eight times as much as during rest. The volume of carbon dioxide given off varies from about half a cubic foot per hour during complete rest to 5 cub. ft. during severe exertion, but averages about o.g cub. ft. per hour, and will reach or exceed' r cub. ft. per hour during even very light exertion. The volume of oxygen consumed is about a seventh greater than that of the carbon dioxide given off. The breathing is regulated, from a nervous centre situated in the medulla oblongata, which is the lowest part of the brain. If this centre is destroyed or injured the breathing stops and death rapidly results. From the respiratory centre rhythmic efferent impulses proceed down the motor nerves supplying the diaphragm, intercostals and other respiratory muscles. Afferent impulses through various nerves may temporarily affect the rhythm of the respiratory centre. Of these afferent impulses by far the most important are those which proceed up the vagus nerve from the lungs themselves. On distention of the lungs with air the inspiratory impulses from the respiratory centre are suddenly arrested or " inhibited "; on the other hand, collapse of the lung strongly excites to inspiratory effort. On section of the vagus nerve these effects disappear, and the breathing becomes less frequent and much more laboured. The vagus nerve is thus the carrier of both inhibitory and exciting stimuli. As the physiological function of breathing is to bring oxygen to and remove carbon dioxide from the blood, it would naturally be expected that breathing would be regulated in accordance with the amount of oxygen required and of carbon dioxide formed; but until quite recently the actual mode of regulation was by no means clear. It was commonly supposed that afferent nervous impulses in some way regulated the otherwise automatic action of the centre, want of oxygen or excess of
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